This application claims the priority of Korean Patent Application No. 2003-95544, filed on Dec. 23, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a flip-chip light emitting diode (FCLED) and a method of manufacturing the same, and more particularly, to an FCLED having an electrode structure with an improved thermal stability and a method of manufacturing the same.
2. Description of the Related Art
An ohmic contact structure between a semiconductor and an electrode is important to realize a light emitting diode, such as a laser diode or a light emitting diode using a nitride semiconductor, for example, a gallium nitride (GaN) semiconductor of generating blue, green, and ultraviolet rays. A conventional GaN based light emitting diode, which is commercially used nowadays, is formed of an insulating sapphire (Al2O3) substrate.
Such a GaN based light emitting diode is divided into a top-emitting light emitting diode (TLED) and a flip-chip light emitting diode (FCLED).
A TLED emits light through an ohmic electrode layer, which contacts a p-type cladding layer.
In addition, a TLED has problems of deteriorated electric characteristics, such as a low current injection and a current spreading due to the characteristics of a p-type cladding layer having a low hole concentration. However, such problems can be solved by developing a transparent ohmic contact electrode having a low sheet resistance.
The TLED uses an oxidized semi-transparent nickel (Ni)/gold (Au) layer as a transition metal based metal layer. In this case, an example of the transition metal is Ni.
Such a Ni based metal layer is used as a semi-transparent ohmic contact layer having low specific-contact resistance of 10−3 to 10−4 Ω/cm2.
When a Ni/Au metal layer is annealed at a temperature of 500 to 600° C. under an oxygen atmosphere, an island shaped nickel oxide (NiO) as a p-type semiconductor oxide is formed on the interface between p-type GaN and Ni due to the low specific-contact resistance, thus a Schottky barrier height (SBH) is lowered. In addition, holes, in other words, majority carriers, are easily supplied to the surface of GaN to increase an effective carrier concentration. On the other hand, when a Ni/Au metal layer is annealed after contacting p-type GaN, a Mg—H metal compound is removed and a reactivation of increasing a magnesium dopant concentration on the surface of GaN occurs. Thus, the effective carrier concentration becomes higher than 1019 on the surface of the p-type GaN, and a tunneling conduction occurs between the p-type GaN and an electrode layer, resulting in the generation of an ohmic conductive characteristic.
In this case, a TLED using a semi-transparent electrode layer of Ni/Au has a low light utility efficiency, thus it is difficult to realize a light emitting diode of high capacity and luminance.
Nowadays, an FCLED using a material having high reflectivity, such as silver (Ag), silver oxide (Ag2O), aluminum (Al), or rhodium (Rh), is developed to realize a light emitting diode of high capacity and luminance.
Such a material has high reflectivity to generate a high light emitting efficiency for a short time; however, such a material has a low work function, resulting in the difficulty in forming an ohmic contact having low resistivity. As a result, the lifespan of a light emitting diode is reduced. In addition, such a material cannot be adhered to GaN well, thus reliability of the light emitting diode is deteriorated.
More specifically, Al has a low work function and generates an aluminum nitride (AlN) even at a low temperature, thus it is difficult to form an ohmic contact with p-type GaN.
Rh has a relatively high work function of about 5 eV and generates gallide as a gallium compound after annealing, thus it is possible to form an excellent ohmic contact electrode on p-type GaN; however, Rh has a low reflectivity against light compared to Al and Ag.
In addition, Ag has a high reflectivity and may form an excellent ohmic contact; however, Ag cannot form an excellent layer due to a thermal instability. In other words, a Ag layer is thermally unstable, thus generates an agglomeration phenomenon at an early stage of annealing. In addition, the Ag layer is changed to void, hillock, and island shapes at a final stage of annealing, resulting in the deterioration of electric and optical characteristics.
Recently, studies of developing an ohmic contact layer of providing a high reflectivity while having a low specific-contact resistance are performed in order to use a light emitting diode as a large area and high capacity light emitting diode, such as a back light of a vehicle and a household lighting.
A Ni/Al structure and a Ni/Ag structure has been provided; however, such a structure cannot form an ohmic contact easily and requires a high operation voltage when operating a light emitting diode, thus generates a large amount of heat.
In addition, US Patent No. 2002/0171087 A1 discloses a Ni/Ag electrode structure and a Au/NiOx/Al electrode structure. However, such an electrode structure has a low adherence and a low light emitting efficiency doe to scattered reflection.